CO2/brine interfacial tension for geological CO2 storage: A systematic review

Given that the interfacial tension (IFT) of the CO2/brine system is a critical parameter in geological CO2 storage (GCS), this paper presents a systematic review of the present studies on the IFT of CO2/brine system, mainly including experimental methods, data and influencing factors.IFT is caused by an imbalance of molecular forces at the interface of two immiscible fluids, and the pendant drop method is considered the most convenient method to determine its value. The axisymmetric drop shape analysis (ADSA) theory derived from the Young-Laplace equation is regarded as the most advanced and accurate theory for the analysis of the results of the pendant drop measurements. The IFT of the CO2/brine system is determined by the temperature, pressure, salt species and salinity of the brine and the impurity of the CO2 phase. The results indicated that the greater solubility of CO2 caused by increasing temperature increase the density difference between brine and CO2, leading to an increase in the IFT. Under low-pressure conditions, the IFT decreases suddenly due to the increase in the CO2 solubility and the CO2 density with pressure. However, after reaching the supercritical state; CO2 solubility and density hardly change with pressure, thus, IFT does not depend on the pressure. When ions are dissolved in water, the combined effect of decreased solubility of CO2 in the aqueous phase, and enhanced electrostatic force and interfacial ionic strength gradient leads to a positive effect of the ion concentration on the IFT. The introduction of impurity gases lighter than CO2 can enhance the IFT by increasing the density difference between CO2 and brine. The findings of this study can help to better understand the IFT of CO2/brine systems used for geological CO2 storage.